Ranged combat is why. Moving on a left click would work better now that left click doesn't handle the tooltips -- previously we had to do right-click. But honestly this is a rat-hole in my opinion, a long slide into a lot of unpleasant stuff that can't be solved well. Having all the enemies not only be ranged, but long ranged, and in numbers, just does not work for mouse movement.
It sounds good until you really think it through, or in our case try it. Giving a "simple move order" that takes you into a location that gets you shot up is Not Fun with capital letters. It leads to doing lots of tiny clicks here and there, not doing the longer clicks like you might expect.
Only allowing a single step per click would of course solve the problems, but I can't imagine that anyone really wants that? And since you can only move in cardinal directions, if someone clicks at an angle, what does that mean? Go which direction first? That decision is pretty important, because then that can determine if you get shot or not on that movement. So then we wind up needing, what, some sort of predictive AI to make sure the pathing is optimal in terms of not getting shot. Or players need to remember to not click diagonals.
Plus then we have this huge visual indicator all over the place any time your mouse cursor is anywhere in range of clicking, or else the visual indicator is so small people can't see it. Or there's no visual indicator, and then it's not clear.
See what I mean about a rat hole?
One thing I could see working, if someone just hates touching the keyboard, would be to let players left click on the map tiles directly to the NSEW and have them move to that tile. Or click and hold to move in that direction. Basically substituting for the WASD keys, but only in a really limited, clear zone. Something along those lines.
But seriously, WASD + Mouse is just super common in games. Terraria. Every FPS. Etc. Surely this is not like Valley 2 when everyone got all upset about no mouse support. I mean, I hate walking into something like that, and this has nothing to do with "playing optimally" this time. It's a matter of how easy it is for people to shoot themselves in the foot and then ragequit for something that is superfluous in the first place.